The interferometer is a device which can be used for optically determining a distance moved and/or a speed of movement and thus, indirectly, a relative distance. Laser interferometers are used for the accurate determination of length and the calibration of precision length measuring equipment. Although such devices have been known and used in the field for some time, the degree of precision attainable has increased considerably in recent years. Numerous innovations have each contributed to additional increases in resolution and reliability of such devices, and have expanded the scope of useful applications. However, the need to accurately measure smaller and smaller distances in both the field of metrology and in the manufacturing sector has increased at least as rapidly as any advances in the ability to conveniently measure those distances.
Many of the advances in the field have involved improvements in the electronic apparatus for detecting and analyzing fringes created by phase differences between a measurement laser beam and a reference laser beam, and thus for detecting and analyzing indications of movements resulting in changes in length of a path traveled by the measurement laser beam. However, some of these improvements have also involved the optical aspects of the laser interferometer.
Two of the most common types of interferometer optic arrangements known in the field are the linear interferometer and the plane mirror interferometer. The linear interferometer compares a measurement laser beam, which has been directed to a movable target and reflected to a detector, to a reference laser beam which has traveled a fixed distance. Doppler shift of the measurement laser beam caused by any movement of the target results in detectable phase distinctions between the measurement laser beam and the reference laser beam. The typical resolution attainable by a linear interferometer is one quarter of a wave length of the measurement laser beam.
The plane mirror interferometer has provided a significant variation of the laser interferometer. Briefly, the plane mirror interferometer provides for an increase in resolution by directing the measurement laser beam such that it travels the distance between the interferometer optics and the moving target four times, twice upon being directed toward the target and twice upon being reflected from the target back toward the interferometer optics, thus providing for two Doppler shifts of the measurement laser beam. This increases any phase difference between the measurement laser beam and the reference laser beam for any given movement of the target, and thus provides for detection of target movement at an improved level of resolution. The typical resolution attainable by a plane mirror interferometer is one eighth of a wave length of the measurement laser beam. A good description of the common plane mirror interferometer is to be found in U.S. Pat. No. 4,334,778, issued to Pardue et al., which patent teaches a variation of the plane mirror interferometer adapted to the specific purpose of measuring a distance between two opposed surfaces.
Although the several improvements in the field have resulted in a considerably increased potential resolution, there remains a need for yet further improvement. While the idea of increasing resolution by means of an increased number of passes of the measurement laser beam between the interferometer optics and a target has been the subject of some investigation, methods for accomplishing this goal which have been tried have been less than ideally successful, due primarily to the fact that the increased quantity of optical components involved has introduced unwanted complexity and an increased error factor. Also, in all such attempts with which the inventor is familiar, at least some of the added optical components have not been incorporated as a single unit into the primary optical unit. Of course, this has resulted in an increased potential for misalignment and in resultant reduced resolution and/or reduced reliability. Clearly, there exists a need for a means to increase the obtainable resolution of a laser interferometer by increasing the passes of the measurement beam which is reliable and accurate in operation and which does not introduce unwanted complexity or error.
All of the prior art interferometers within the inventor's knowledge have incorporated optics which were either incapable of the resolution attainable through use of the present invention or were prone to misalignment or other error producing aberrations.
No prior art interferometer to the inventor's knowledge has successfully provided resolution on the order of one sixteenth of a wave length without itself being a source of considerable potential error. All successful laser interferometer optics to date have provided fewer than four Doppler shifts of the measurement beam, or else have not taken full advantage of any additional Doppler shifts because of a less than ideal optical arrangement.